One type of antenna is a quadrifilar helix antenna. The quadrifilar helix antenna is also sometimes called a quadrature helix antenna or four-wire helical antenna.
The quadrifilar helix antenna is described in Patent Documents 1 and 2, for example.
In Patent Document 1, a quadrifilar helix antenna device is disclosed. The quadrifilar helix antenna device of Patent Document 1 has the structure of supplying power to each helical antenna element in a non-contact manner. Moreover, in Patent Document 1, a 90° hybrid and a 180° hybrid are described. A hybrid is called a phase shifter, a mixer, a coupler, or a multiplexer, or is also sometimes called a hybrid phase shifter, a hybrid mixer, or a hybrid coupler.
Also in Patent Document 2, a quadrifilar helix antenna device is disclosed. The quadrifilar helix antenna device of Patent Document 2 includes the structure of switching between a first mode, which is a mode compatible with circularly polarized waves, and a second mode, which is a mode compatible with directly polarized waves, with a switch in each system. The quadrifilar helix antenna device connects a delay line to each helical antenna element to change the mode by switching from the first mode to the second mode with the switch in each system.
Further, related technologies are described also in Patent Documents 3 and 4.
In Patent Document 3, a fading elimination method for a single antenna for multipath generated on a sea surface. In Patent Document 3, there is disclosed a demultiplexer/multiplexer based on characteristics of the multipath generated on the sea surface. Moreover, in Patent Document 3, there are described a phase shifter (variable phase shifter) capable of adjusting an amount of phase shift, and an attenuator (variable attenuator) capable of adjusting an attenuance. Further, in Patent Document 3, there is described a combination circuit (corresponding to 180° combiner) of a phase shifter and a synthesizer (mixer), which performs phase shift by 180° and then combining. In this method, a hybrid coupler separates an antenna wave into a signal wave obtained by multiplexing a normally rotated direct wave (1) of a circularly polarized wave and a normally rotated reflected wave (2) of the circularly polarized wave, and a reversely rotated reflected wave (3) of the circularly polarized wave. Next, the attenuator and the phase shifter adjust the reversely rotated reflected wave (3) of the circularly polarized wave to an opposite phase and the same amplitude of the normally rotated reflected wave (2) of the circularly polarized wave. Finally, the synthesizer multiplexes the signal wave obtained by multiplexing the normally rotated direct wave (1) of the circularly polarized wave and the normally rotated reflected wave (2) of the circularly polarized wave, and a signal wave obtained by adjusting the reversely rotated reflected wave (3) of the polarized wave. As a result, the reflected wave generated on the sea surface can be ideally eliminated, and only the normally rotated direct wave (1) of the circularly polarized wave is obtained. This method is not a measure against fading for a quadrifilar helix antenna device. Moreover, the method is not a measure against fading generated on a ground surface or other surface.
Also in Patent Document 4, a demultiplexer/multiplexer is disclosed. In Patent Document 4, the demultiplexer/multiplexer includes one phase shifter (variable phase shifter), a four-beam changeover switch, and one combiner/splitter.